[Analyzer] Fix for `ExprEngine::computeObjectUnderConstruction()` for base and...

[Analyzer] Fix for `ExprEngine::computeObjectUnderConstruction()` for base and delegating consturctor initializers

For /C++/ constructor initializers `ExprEngine:computeUnderConstruction()`
asserts that they are all member initializers. This is not neccessarily
true when this function is used to get the return value for the
construction context thus attempts to fetch return values of base and
delegating constructor initializers result in assertions. This small
patch fixes this issue.

Differential Revision: https://reviews.llvm.org/D85351